Our objectives are severalfold: A. To produce radiation resistant mutants from a stable strain of mammalian cells growing in various environments of radiation. B. To identify, describe and assess biochemical, biophysical, and cytological differences between the mutants and the parent strain. C. To quantitate the overall mutation rate for radiation resistance as a function of various parameters such as dose rate, temperature, and time schedule for dose delivery including the simulation of a fractionated radiation therapy regimen, and to examine some of the kinetics of the production of specific mutations if warrated by the data obtained.